EP0159546B1

EP0159546B1 - Digital graphic equalizer

Info

Publication number: EP0159546B1
Application number: EP85103312A
Authority: EP
Inventors: Naohisa Kitazato; Osamu Hamada
Original assignee: Sony Corp
Current assignee: Sony Corp
Priority date: 1984-03-24
Filing date: 1985-03-21
Publication date: 1989-05-31
Also published as: AU579853B2; EP0159546A1; DE3570825D1; AU4016985A; JPH0663777B2; KR850007166A; US4661982A; KR930009034B1; JPS60199212A; CA1230828A

Description

Field of the invention

This invention relates generally to a digital graphic equalizer and particularly to a digital graphic equalizer which is used for processing a digitized audio signal by a desired frequency characteristic.
GB-A-20 68 678 discloses an automatic closed-loop system for providing tone equalization in which either a noise generator, for example a so-called pink noise generator, or an audio signal from a preamplifier are supplied to a digitally controlled multi-band equalizer unit for imposing a specific filter characteristic which signal is then emitted to an environment, for example a room, via a speaker system for accommodating the acoustical effects of the listening environment and for achieving a desired flat frequency response. The equalizer system is provided with means, for example with a microphone followed by a preamplifier, for detecting the measuring noise signal the frequency response of which has been changed in said listening environment. The detected signal is supplied to a second digitally controlled signal processor means for producing a plurality of frequency divided signals, and a microprocessor implemented spectrum analyzer device receives said plurality of frequency divided signals so as to analyze the frequency response of the detected measuring noise signal. The frequency characteristic can be adjusted via a keyboard and a graphic display is provided for giving the user an indication about the adjusted frequency response of an audio equipment provided in said environment.
Similar digitally controlled graphic equalizers including analogue filter elements are described in Audio Engineering Society Reprint, presented at the 67th Convention, 31st October to 3rd November, 1980, pages 1 to 16 and in Journal of the Audio Engineering Society, Vol. 31, No. 1/2, 1983, pages 25-28.
Following suggestions given by Y. Hirata in cooperation with Sony Corporation in Journal of the Audio Engineering Society, Vol. 29, No. 5, 1981, pages 333 to 337, it is also known to digitize an audio signal and to arithmetically process such digital signal in accordance with the frequency characteristics of various filters.
Figure 1 illustrates an example of such prior art digital graphic equalizer. Referring to Figure 1, a digital audio signal applied to an input terminal 1 is delivered through a multiplier 2 having a multiplication coefficient K, an input adder 3 and an output adder 4 to an output terminal 5. The signal between the input and. output adders 3 and 4 is supplied to a circuit formed by connecting in cascade delay circuits 6₁, 6₂ ... 6_n each of which has a delay amount Z-'. The outputs from these delay circuits 6₁ to 6_n are respectively supplied through multipliers 7₁, 7₂... 7_n having multiplication coefficients a₁, a₂ ... an to the above-described output adder 4. Further, the outputs from the delay circuits 6₁, 6₂ ... 6_n are respectively supplied through multipliers 8₁, 8₂ ... 8_n having multiplication coefficients b₁, b₂ ... b_n to the above-described input adder 3.
In this circuit arrangement, the transfer function from the input terminal 1 to the output terminal 5 is expressed as
Accordingly, various filter characteristics or filter characteristics of desired filters such as a low-pass filter, a high-pass filter, a band-pass filter or the like can be obtained by varying the above-mentioned coefficients K, a₁ to an and b₁ to b_n.
As a result, if the above-mentioned circuit is connected in cascade in a plurality of stages (for example, 8 stages), and the characteristics of the respective circuits are made such that, as shown in Figure 2, peak characteristics are obtained at desired resonance frequencies f, to f_a and the levels at the resonance frequencies f, to f₈ can be adjusted independently, a so-called digital graphic equalizer can be formed.
With a digital graphic equalizer of this type, since all the signal processings are carried out in a digital fashion, when a PCM (pulse code modulated) audio signal and the like signals are processed, the tone quality is not deteriorated unlike the prior art analog signal processing. The signal processing being carried out accurately and easily.
For example, the above-described digital graphic equalizer is used to obtain an equalizer frequency characteristic which is flat in accordance with a listening environment such as a listening room in which an apparatus is installed. However, since the interior room of a vehicle, a listening room of a house and so on are different in acoustic characteristic, different equalizer characteristics are required.
Therefore, it is considered that the acoustic characteristic of the listening environment in which the apparatus is installed is measured, a sound field compensating characteristic for the listening environment is provided and then this . compensating characteristic is automatically set at the above-described digital graphic equalizer.
However, in order to measure the acoustic characteristic, a so-called pink noise (noise with frequency characteristic the frequency level of which is lowered with a ratio of 3 dB/oct.) is emitted from a loudspeaker and then collected by a microphone provided at a predetermined place and the collected signal is analyzed by a spectrum analyzer. Of course, this needs a pink noise generator, a spectrum analyzer and so on and the circuit arrangement becomes large and complex. Further, it is troublesome to set the measured compensating amount at the digital graphic equalizer and the circuit arrangement of a control circuit becomes more complicated.

Objects and summary of the invention

Accordingly, it is an object of this invention to provide an improved digital graphic equalizer.
It is another object of this invention to provide a digital graphic equalizer having a digital signal processor to digitally process a pink noise generator and a spectrum analyzer.
It is a further object of this invention to provide a digital graphic equalizer having at least two digital signal processing units in which a digital signal is delayed by a predetermined timing, this digital signal is multiplied with a desired coefficient and this signal with the coefficient multiplied is added to the above original digital signal in feedforward mode or feedback mode to produce a desired filter characteristic, wherein the first digital signal processing unit produces a pink noise with the predetermined filter characteristic and the second digital signal processing circuit uses the desired filter characteristic to carry out the spectrum analysis of the signal to thereby produce a sound field compensating characteristic.
The present invention provides for a digital graphic equalizer comprising a measuring noise generator producing a measuring noise with specific frequency characteristic, means for emitting said measuring noise in a listening environment, means for detecting said measuring noise, the frequency response of which is changed in said listening environment, analog-to-digital converter means supplied with said detected measuring noise and producing digital signals representing the amplitude of said detected measuring noise, spectrum analyzer means for receiving said digital signals from said analog-to-digital converter means for analysing the frequency response of the detected noise which is characterized in that said measuring noise generator is associated with one channel of a two-channel audio-system and comprises a pulse sequence generator for producing a sequence of pulse signals and first digital signal processor means supplied with said sequence of pulse signals and digitally filtering the sequence of said pulse signals which processed pulse signals are then supplied to a digital-to-analog converter so as to produce said measuring noise, and in that said spectrum analyser means is associated with the other channel of said audio-system and consists of a second digital signal processor means supplied with the digital signal from said analog-to-digital converter means and producing a plurality of frequency-divided signals for analysing the frequency response of the detected noise.
Other objects, features and advantages of the present invention will become apparent from the following description taken in conjunction with the attached drawings.

Brief description of the drawings

Figure 1 is a circuit block diagram showing an example of the prior art digital graphic equalizer;
Figure 2 is a graph showing a control characteristic of the digital graphic equalizer of Figure 1;
Figure 3 is a circuit block diagram showing an embodiment of the digital graphic equalizer according to this invention; and
Figures 4 to 6 are respectively circuit block diagrams each showing a main part of the embodiment of the present invention.

Description of the preferred embodiment

Figure 3 schematically illustrates an embodiment of the digital graphic equalizer according to this invention. In Figure 3, left and right stereophonic audio signals, for example, applied to input terminals 11 L and 11 R are respectively supplied to A/D (analog-to-digital) converters 12L and 12R and the digitized signals therefrom are respectively supplied to digital signal processing units (hereinafter simply referred to as DSPs) 13L and 13R. By the way, each of the DSPs 13L and 13R incorporates in practice though not shown a multiplier, an adder, a shift register and so on.
A digital signal indicative of the adjusting position from each of volume adjusters 14₁, 14₂ ... 14₈, each of which serves as means for adjusting, for example, an equalizer characteristic, is supplied to a host CPU (central processing unit) 15. In accordance with each of the signals from these volume adjusters 14, to 14a, the coefficient (K, a and b) necessary for the equalizing characteristics is retrieved from a ROM (read only memory) 16 which serves as a coefficient table and the coefficient is supplied to the DSPs 13L and 13R, respectively.
In each of the DSPs 13L and 13R, the calculation is carried out in accordance with a command memory stored therein and the calculation corresponding to a block diagram as, for example, shown in Figure 4 is carried out. The circuit of Figure 4 is equivalent to the circuit formed by connecting in cascade the circuits of Figure 1 in 8 stages, in which frequency characteristics are compensated for with 9600 Hz, 4800 Hz, 2400 Hz, 1200 Hz, 600 Hz, 300 Hz, 150 Hz and 75 Hz supplied thereto as varying points from the input side. The compensating levels are controlled by the coefficients (K, a and b) obtained by the values from the volume adjusters 14, to 14₈.
Turning back to Figure 3, the output signals from the DSPs 13L and 13R are respectively supplied to D/A (digital-to-analog) converters 17L and 17R and the converted analog signals therefrom are respectively delivered to output terminals 18L and 18R.
Further, in this digital graphic equalizer, a RAM (random access memory) 19 is provided in association with the host CPU 15, and also push buttons 14₁, to 14₁₃ for presetting are provided to select a desired address region of the RAM 19. Together with the push buttons 14₁, to 14i3, there is provided a push button 14S for set-controlling upon presetting the sound field characteristic.
When the set-controlling push button 14S is depressed, the mode for measuring the sound field is carried out in each of the DSPs 13L and 13R.
In the sound field characteristic measuring mode, the first DSP 13L generates pink noise. Figure 5 is a circuit block diagram showing an example therefor. In Figure 5, an M-sequence code generator generates a signal of a so-called M-sequence code noise and for producing the desired pink noise this signal is supplied to a filter the frequency response of which is -3 dB/oct. In this case, the M-sequence code generator is operated to generate the pink noise by controlling a shift register (not shown) of the first DSP 13L and the filter having the frequency response of -3 dB/oct. is formed by connecting, for example, 9 filters in cascade similarly to the above-described graphic equalizer.
The pink noise thus generated is supplied to and converted to the analog signal by the D/A converter 17L. This analog signal is supplied through an output terminal 18L to a loudspeaker (not shown) from which it is emitted.
The second DSP 13R carries out the spectrum analysis. Figure 6 is a circuit block diagram showing the circuit therefor. In Figure 6, the signal collected by a microphone (not shown) is supplied through the input terminal 11R to the A/D converter 12R and converted to a digital audio signal. The digital audio signal is supplied to the second DSP 13R.
In the DSP 13R, band-pass filters (BPFs) with frequencies corresponding to those of the frequencies f, to f₈ are provided in parallel to one another. The outputs from these band-pass filters are respectively squared and then processed. In other words, when the digital values contain positive and negative data, the absolute value is obtained by squaring the digital values. In this case, the band-pass filter of Figure 6 is formed similar to the above-described graphic equalizer and the product is carried out by using a shift register, the adder and so on as those incorporated in the DSP 13R.
When one of the product values reaches a predetermined value, all the product calculations are stopped and each of the product values at that time is detected. In this case, each product value corresponds to the acoustic characteristic of the place where the loudspeaker and the microphone are located.
These product values are supplied to the host CPU 15 and the host CPU 15 carries out a logarithmic compression and a compensating spectrum characteristic conversion. The converted values are written in the RAM 19 at its addresses selected by the push buttons 14₁, to 14₁₃. The values to be written correspond to the values from the volume adjusters 14, to 14₈.
As described above, the acoustic characteristic is measured and the value corresponding to the compensating characteristic is written in the RAM 19. Accordingly, when the push buttons 14" to 14₁₃ are depressed next, the values written in the RAM 19 at the respective addresses specified by the push buttons 14₁₁ to 14₁₃ are read out therefrom and then used to retrieve the coefficient table of the ROM 16. The derived coefficients therefrom are set to the DSPs 13L and 13R, and the DSPs 13L and 13R carry out the graphic equalizer operation according to the sound field compensating characteristic.
According to the digital graphic equalizer of this invention, since the DSPs 13L and 13R are used to generate the pink noise and to carry out the spectrum analysis, it is possible to measure the acoustic characteristic without providing any special or new circuit arrangement., Further, the compensating characteristic is automatically set to the RAM 19 and then read out therefrom to thereby carry out the sound field compensation with ease. Furthermore, for example, since the sound field compensating characteristic of the interior room of a vehicle is written in the RAM 19 at its address selected by the push button 14₁₁ and the sound field compensating characteristic of the room is written in the RAM 19 at its addresses selected by the push button 14₁₂, it is possible to obtain the desired characteristics by only depressing the push button 14" or 14₁₂ as required, thus the usefulness of the digital graphic equalizer for users is additionally improved.
Further, with the above-mentioned digital graphic equalizer, the signal processing is all carried out in a digital signal format so that the control by the digital graphic equalizer is accurate. Furthermore, since the signal processing of the graphic equalizer itself, the pink noise generation and the spectrum analysis are all carried out by the same hardware, the circuit arrangement can be made small in scale and simple in construction. In addition, since the digital graphic equalizer of this invention is advantageous in respect of space and manufacturing cost and at the same time, since the content of the signal processing is determined by software, the digital graphic equalizer of this invention can be designed with much flexibility and is suitable for mass-production.

Claims

1. A digital graphic equalizer comprising:

-a measuring noise generator (13L) producing a measuring noise with specific frequency charac- . teristic;

-means (17L, 18L) for emitting said measuring noise in a listing environment;

-means for detecting said measuring noise, the frequency response of which is changed in said listing environment;

-analog-to-digital converter means (12L) supplied with said detected measuring noise and producing digital signals representing the amplitude of said detected measuring noise;

-spectrum analyser means (13R, 15, 16, 19) for receiving said digital signals from said analog-to-digital converter means for analysing the frequency response of the detected noise characterized in that

-said measuring noise generator is associated with one channel of a two-channel audio-system and comprises a pulse sequence generator for producing a sequence of pulse signals and first digital signal processor means (Figure 4) supplied with said sequence of said pulse signals and digitally filtering the sequence of said pulse signals, which processed pulse signals are then supplied to a digital-to-analog converter (17L) so as to produce said measuring noise, and in that

-said spectrum analyser means is associated with the other channel of said audio-system and consists of a second digital signal processor means supplied with the digital signal from said analog-to-digital converter means (12R) and producing a plurality of frequency-divided signals for analysing the frequency response of the detected noise.

2. A digital graphic equalizer according to claim 1, in which said first digital signal processor means produces a measuring noise the frequency response of which equals -3 dB/oct.

3. A digital graphic equalizer according to claim 2, in which said measuring noise is a pink noise.

4. A digital graphic equalizer according to claim 3, in which said pulse-sequence generator is a M-sequence code generator.

5. A digital graphic equalizer according to claim 1, in which said first digital signal processor means comprises cascade-connected low-pass filters.

6. A digital graphic equalizer according to claim 5, in which each of said low-pass filters comprises an input adder having first and second input terminals and an output terminal, the first input terminal of which is supplied with the output of said M-sequence code generator; an output adder having first and second input terminals and an output terminal; and delay circuit means having an input terminal connected to the output terminal of said input adder and the first input terminal of said output adder, the output terminal of which is connected to the second terminals of said input and output adders through first and second multipliers, respectively.

7. A digital graphic equalizer according to claim 1, in which said spectrum analyzer means includes a logarithmic compressor.

8. A digital graphic equalizer according to claim 1, further including a memory (19) for storing said compensating characteristics.

9. A digital graphic equalizer according to claim 8, in which said memory is a random access memory.